The present invention relates to a pyroelectric device used as an infrared-ray sensor and heat sensitive device.
A pyroelectric device can be used as an infrared-ray sensor using a pyroelectric element composed of a material such as, for example, PVDF (polyvinylidene fluoride), PZT (lead zirconate titanate) or the like, or can be used as a sensing device such as a flame sensor disposed in a tunnel or the like to detect infrared rays of flame when fire is caused, or can be used as an intrusion detector used in a security system for detecting infrared rays from a human body.
Conventionally, as shown in FIG. 6, a pyroelectric device is arranged such that a pyroelectric member 40 and a printed board 41 are disposed on a base comprising stem 42 composed of metal or the like, and are sealed by a frame member comprising a can 43 composed of a steel plate or the like covering the upper periphery of the stem 42. The printed board 41 includes circuit elements 61 such as a field effect transistor (FET) or the like mounted thereon to output the intensity of infrared rays detected by the pyroelectric member 40 in the form of an electric signal. More specifically, the can 43 is provided with a light transmitting window hole 44 through the ceiling thereof so that unillustrated infrared rays emitted from the flame of, for example, a fire or the like are irradiated to the pyroelectric member 40 through the window hole 44. A filter 45 composed of, for example, silicon which is excellent in infrared ray transmittance is disposed over the window hole 44 to seal the same and enable the infrared rays to be transmitted therethrough. The sealing by and the bonding of the filter 45 is performed by forming an adhesive-bonding portion 46 with an adhesive such as an epoxy resin or the like.
Although the casing of the pyroelectric device is composed of the can 43 and stem 42 as described above, the sealed type pyroelectric device is formed in such a manner that the can 43 and stem 42 are fused or adhesive-bonded to each other at the sealing portion 47 along the periphery of the stem. As shown in FIG. 6, three lead terminals, that is, a ground terminal. 48, a source terminal 49 and a drain terminal 50 extend from the stem 42 and the aforesaid electric signal is output to an unillustrated main electric circuit through these lead terminals. In this case, although the ground terminal 48 is directly connected to the stem 42, the two other terminals, that is, the source terminal 49 and drain terminal 50 are fixed to the stem 42 in a sealed state through insulating members 51 so as to be insulated from the stem 42.
Arrangement of the pyroelectric member 40 will be described in detail with reference to the enlarged cross sectional view shown in FIG. 7. The pyroelectric member 40 is disposed on the printed board 41 through a base member 58, and keyhole-shaped electrodes 52 and 53 each composed of a protecting rectangular portion and a disc-shaped portion are formed respectively on the upper and lower surfaces of the pyroelectric member 40 by vapor deposition or the like. The disc-shaped portions 54 and 55 of these electrodes are disposed to confront to each other across the pyroelectric member 40, and the projecting portions 56 and 57 thereof are disposed so as to extend from the disc-shaped portions 54 and 55 to the right and left sides of the pyroelectric member 40, respectively. The electrodes 52 and 53 are composed of an infrared ray absorbing material such as nickel chromium alloy, gold black or the like.
The pyroelectric member 40, base member 58 and printed board 41 are bonded and fixed to each other through a suitable material, respectively, and electrically conductive adhesives 59 and 60 are applied between the electrodes 52 and 53 on the upper and lower surfaces of the pyroelectric member 40 and unillustrated circuit patterns of the printed board 41 so that electric conductivity is established therebetween. An electrically conductive adhesive mixed with silver filler or the like is used as the electrically conductive adhesives 59 and 60.
When the aforesaid pyroelectric device is used as, for example, a flame sensor, it is required to operate normally as well as safely and securely in environments having a wide range of humidities, having corrosive gases and having a wide range of temperatures. This is because pyroelectric devices are used in severe environmental conditions in the indoors and outdoors such as, for example, in a factory, parking area, hot-spring resort, tunnel or the like. When an accelerated operation test is executed in correction with the conventional pyroelectric device arranged as described above to confirm its operation to satisfy the above requirements, since the can 43 is composed of a steel plate and the filter is composed of silicon, a problem arises in that the surface of the can 43 rusts and further the occurrence of the rust is naturally accelerated under the existence of the corrosive gases. In addition, a gap is formed in the adhesive-bonding portion 46 adjacent to the window hole 44 and the filter 45 cracks due, to the difference in thermal expansion coefficients thereof.
Further, the pyroelectric device includes the filter 45 fixed over the window hole 44 of the can 43 by the resin adhesive-bonding portion 46. Since the bonding portion 46 is composed of the resin, a gap is liable to be formed due to the ventilating property and deterioration of the resin, although this is dependent on the characteristics of the resin, and thus the functional life of the bonding portion 46 in providing an airtight seal is very short. That is, a problem arises in that moisture contained in the outside air or corrosive gases penetrates through the bonding portion 46 and corrodes the pyroelectric member 40 and circuit elements 61 to lower the reliability of the pyroelectric device.
Further, in this type of pyroelectric device, the electrodes 52 and 53 formed on the surfaces of the pyroelectric member 40 have a function of electrically connecting to the pyroelectric member 40 as well as the another function of containing infrared rays incident on the pyroelectric member 40. More Specifically, it is ideal that the electrode 52, as a light receiving surface, absorbs the infrared rays without reflecting the same and the electrode 53 on the backside thereof reflects the infrared rays without causing the same to be transmitted therethrough so as to improve the infrared racy absorbing efficiency to the pyroelectric member 40. Therefore, the electrodes 52 and 53 are individually formed to a predetermined optimum thickness.
The electrically conductive adhesives 59 and 60 are applied to the electrodes 52 and 53, respectively, and when the adhesives are cured, a tension is applied to the electrodes 52 and 53. Since the electrodes 52 and 53 are formed thin for the purpose of absorbing infrared rays, and thus when the thin and slender electrodes 52 and 53 are subjected to corrosion, vibration or the like, they may be easily cut off at the boundary between them and the adhesives 59 and 60. In particular, when the pyroelectric device is used as a flame sensor for detecting infrared rays, severe regulations regarding durability are applicable to the pyroelectric device, and it may be installed at a place with very bad environmental conditions under which the pyroelectric device is damaged.